SAN JOSE, Calif. — Genetic data are beginning to reveal how the Ebola virus causing the epidemic in Western Africa is evolving.

Scientists have deciphered the entire catalog of genetic data for 96 Ebola viruses taken from patients infected between June and September 2014. The results show that one particular clade, or type of the virus, is dominant among patients in Sierra Leone, suggesting that two other clades that dominated early on in the outbreak have died out. This third clade appears to have evolved starting with a single mutation in the genetic catalog, or genome, of the virus, said Stephen Gire of Harvard University and the Broad Institute in Cambridge, Mass. He presented the preliminary findings February 14 at the annual meeting of the American Association for the Advancement of Science.